Beta catenin (also known as cadherin-associated protein and β-catenin), is a member of the catenin family of cytosolic proteins. β-catenin is encoded by the CTNNB1 gene.
β-catenin is a pivotal player in the Wnt/Wg signaling pathway, mediators of several developmental processes. In the absence of Wnt, glycogen synthase kinase 3 (GSK-3β), a serine/threonine protein kinase constitutively phosphorylates the β-catenin protein. When Wnt is present and binds to any of the family members of the frizzled receptors (Fz), an intracellular signaling protein known as dishevelled (Dsh) is recruited to the membrane and phosphorylated. GSK-3β is inhibited by the activation of Dsh. As a result, β-catenin levels increase in the cytosol and are translocated into the nucleus to perform a variety of functions. β-catenin acts together with the transcription factors TCF and LEF to activate specific target genes involved in different processes.
β-catenin undergoes phosphorylation upon growth factor stimulation resulting in reduced cell adhesion, thereby functioning as a component of adherin junctions which are multiprotein complexes that mediate cell adhesion, cell-cell communication and cytoskeletal anchoring. (Willert et al., 1998, Curr. Opin. Genet. Dev. 8:95-102).
Thompson et al. suggest that β-catenin plays an important role in various aspects of liver biology including liver development (both embryonic and postnatal), liver regeneration following partial hepatectomy, hepatocyte growth factor (HGF)-induced hepatomegaly, liver zonation, and pathogenesis of liver cancer. (Thompson M D., 2007, Hepatology May; 45(5):1298-305).
Wang et al. (2008) have shown that β-catenin can function as an oncogene. (Wang et al., 2008, Cancer Epidemiol. Biomarkers Prev. 17 (8):2101-8). In patients with basal cell carcinoma an increased level in β-catenin is present and leads to the increase in proliferation of related tumors. Mutations in the β-catenin gene are a cause of colorectal cancer (CRC), pilomatrixoma (PTR), medulloblastoma (MDB), hepatoblastoma, and ovarian cancer.
The role of β-catenin in the development of colorectal cancer has been shown to be regulated by the expression product of the APC (adenomatous polyposis of the colon) gene, a tumor suppressor. (Korinek et al., Science, 1997, 275:1784-1787; Morin et al., Science, 1997, 275:1787-1790). The APC protein normally binds β-catenin in conjunction with TCF/LEF forming a transcription factor complex. Morin et al. (Morin et al., Science, 1997, 275:1787-1790) report that APC protein down-regulates the transcriptional activation mediated by β-catenin and Tcf-4 in colon cancer. Their results indicate that the regulation of β-catenin is critical to APC's tumor suppressive effect and that this regulation can be circumvented by mutations in either APC or β-catenin.
Mutations in the β-catenin gene are either truncations that lead to deletion of part of the N-terminus of β-catenin, or point mutations that affect the serine and threonine residues that are targeted by GSK3α/β or CKIα. These mutant β-catenin proteins are refractory to phosphorylation and thus escape proteasomal degradations. Consequently, β-catenin accumulates within affected cells. Stabilized and nuclear-localized β-catenin is a hallmark of nearly all cases of colon cancer. (Clevers, H., 2006, Cell 127:469-480). Morin et al. demonstrated that mutations of β-catenin that altered phosphorylation sites rendered the cells insensitive to APC-mediated down-regulation of β-catenin and that this disrupted mechanism was critical to colorectal tumorigenesis. (Morin et al., 1997, Science 275:1787-1790).
Other studies also report on the detection of mutations in β-catenin in various cancer cell lines (see e.g., Chan et al., 1999, Nature Genet. 21:410-413; Blaker et al., 1999, Genes Chromosomes Cancer 25:399-402; Sagae et al., 1999, Jpn. J. Cancer Res. 90:510-515; Wang et al., 2008, Cancer Epidemiol. Biomarkers Prev. 17(8):2101-8). Additionally, abnormally high amounts of β-catenin have also been found in melanoma cell lines (see e.g., Rubinfeld et al., 1997, Science, 275:1790-1792).
Likewise other cancers, such as hepatocellular carcinoma (HCC), have also been associated with the Wnt/beta-catenin pathway. HCC is a complex and heterogeneous disease accounting for more than 660,000 new cases per year worldwide. Multiple reports have shown that Wnt signaling components are activated in human HCC patients. Activated Wnt signaling and nuclear beta-catenin correlate with recurrence of disease and poor prognosis (Takigawa et al. 2008, Curr Drug Targets November; 9(11):1013-24). Elevated nuclear beta-catenin staining has been documented in 17-66% of HCC patients (Zulehner et al. 2010, Am J Pathol. January; 176(1):472-81; Yu et al. 2009, J. Hepatol. May; 50(5):948-57). Merck's internal dataset on ˜300 HCC patient tumors generated in collaboration with the Hong Kong University indicates Wnt signaling components are activated in 50% of HCC patients. External data have shown activating beta-catenin mutations in 13-40% of HCC patients, while inactivating Axin 1 or 2 mutations were present in ˜10% of HCC patients (Lee et al. 2006, Frontiers in Bioscience May 1:11:1901-1915).
Preclinical studies provide evidence that activation of the Wnt/beta-catenin pathway is important in the generation and maintenance of HCC. Liver-targeted disruption of APC in mice activates beta-catenin signaling and leads to the formation of HCC (Colnot et al. 2004, Proc Natl Acad Sci USA. December 7; 101(49):17216-21). Although overexpression of a beta-catenin mutant lacking the GSK-32beta phosphorylation sites alone is not sufficient for hepatocarcinogenesis (Harada et al. 2002, Cancer Res. April 1; 62(7):1971-7.), overexpression of tumorigenic mutant beta-catenin has been shown to make mice susceptible to HCC induced by DEN (diethylnitrosamine), a known carcinogen (Nejak-Bowen et al. 2010, Hepatology 2010 May; 51(5):1603-13. Interestingly, 95% of HCC tumors initiated by overexpression of the human Met receptor in mice (Tre-Met transgenic mouse model) harbor beta-catenin activating mutations (Tward et al. 2007, Proc Natl Acad Sci USA. September 11; 104(37):14771-6). This finding reflects the human disease and suggests that the Wnt pathway cooperates with Met signaling during hepatocarcinogenesis. High rates of beta-catenin activating mutations are also found in other transgenic mouse models for HCC (16% beta-catenin mutations in FGF19, 55% in c-Myc and 41% in H-Ras transgenic mice) (Nicholes et al. 2002, Am J. Pathol. June; 160(6):2295-307 de la Coste et al. 1998, Proc Natl Acad Sci USA. July 21; 95(15):8847-51).
Preclinical studies have also shown that beta-catenin is a valid target for HCC. Beta-catenin siRNAs inhibit proliferation and viability of human HCC cell lines (Zeng et al. 2007). Similarly, treatment of human HCC cell lines with an anti-Wnt-1 antibody or TCF4/beta-catenin antagonists induce apoptosis, reduction of c-Myc, cyclin D1 and survivin expression as well as suppress tumor growth in vivo (Wei et al. 2009, Mol Cancer September 24; 8:76; Wei et al. 2010, Int J. Cancer. May 15; 126(10):2426-36.2010).
Hepatocellular carcinoma (HCC) is a common and aggressive cancer for which effective therapies are lacking. The Wnt/beta-catenin pathway is activated in a high proportion of HCC cases (˜50%), frequently owing to mutations in beta-catenin (i.e. CTNNB1) or in the beta-catenin destruction complex (e.g. Axin1). Moreover, the Wnt pathway as a target has proven to be challenging and is currently undruggable by small molecule inhibitors, making beta-catenin an attractive target for an RNAi-based therapeutic approach (Llovet et al. 2008, Hepatology October: 48: 1312-1327).
Alteration of gene expression, specifically CTNNB1 gene expression, through RNA interference (hereinafter “RNAi”) is one approach for meeting this need. RNAi is induced by short single-stranded RNA (“siRNA”) or double-stranded RNA (“dsRNA”) molecules. The short dsRNA molecules, called “short interfering nucleic acids (“siNA”)” or “short interfering RNA” or “siRNA” or “RNAi inhibitors” silence the expression of messenger RNAs (“mRNAs”) that share sequence homology to the siNA. This can occur via cleavage of the mRNA mediated by an endonuclease complex containing a siNA, commonly referred to as an RNA-induced silencing complex (RISC). Cleavage of the target RNA typically takes place in the middle of the region complementary to the guide sequence of the siNA duplex (Elbashir et al., 2001, Genes Dev., 15:188). In addition, RNA interference can also involve small RNA (e.g., micro-RNA or miRNA) mediated gene silencing, presumably through cellular mechanisms that either inhibit translation or that regulate chromatin structure and thereby prevent transcription of target gene sequences (see for example Allshire, 2002, Science, 297:1818-1819; Volpe et al., 2002, Science, 297:1833-1837; Jenuwein, 2002, Science, 297:2215-2218; and Hall et al., 2002, Science, 297:2232-2237). Despite significant advances in the field of RNAi, there remains a need for agents that can inhibit CTNNB1 gene expression and that can treat disease associated with CTNNB1 expression such as cancer.